Less isolated than thought: The Bering Land Bridge long formed a link between horses in North America, where they evolved, and Eurasia, where they were eventually domesticated. This is evident from the analysis of DNA remains from horse fossils. The results show a genetic continuity, say the scientists: There is a connection between the horses, which died out in North America at the end of the last Ice Age, and the animals that were later reintroduced to North America by Europeans.
The ancestors of today’s domestic horses originally developed in North America – this has been known for a long time. It is assumed that the so-called caballine horses then spread to Eurasia about a million years ago over the Ice Age Bering Land Bridge, which has repeatedly served as a link for other animal species. Subsequently, over the millennia, the horses on the new continent formed a population that increasingly began to differ genetically from the animals that remained in North America.
While the Eurasians remained on the road to success and their descendants were later domesticated by humans, the horses of America apparently got into trouble: They disappeared there about 11,000 years ago for reasons that have not yet been clarified. In the context of the prehistory of horses, it was also unclear to what extent the two populations in Eurasia and North America had been isolated during their long coexistence in the mid and late Pleistocene: Were there no further contacts or migrations after the first colonization of the continent in the west?
Searching for clues in fossil DNA
In order to gain insights into the possible connections, scientists working with Alisa Vershinina from the University of California in Santa Cruz have now set out to search for traces in the horse’s genome. “This is the first comprehensive look at the genetics of the former horse populations on both continents,” says Vershinina. As part of the study, she and her colleagues sequenced and analyzed numerous mitochondrial and some nuclear genomes that they isolated from North American and Eurasian horse fossils from the Pleistocene. By comparing the obtained genetic data with previously published horse genomes, the researchers created a genetic pedigree. As they explain, it was based on the localities and genetic characteristics whether and when there were connections between the horse populations.
First of all, the researchers were able to confirm that the Eurasian horse populations originally split off from those in North America – their ancestral continent – in the time window from around a million to 800,000 years ago. The genetic analyzes also reflected that after the separation there were at least two bidirectional gene flows in two time windows 875,000 to 625,000 years ago and 200,000 to 50,000 years ago. In plain language this means: The horse populations migrated back and forth on the Bering Land Bridge, which had fallen dry during these times, and mixed, which left traces in the genome of the animals on both continents. “With the data from the mitochondrial and nuclear genomes, we were now able to see that the horses not only spread between the continents, but also crossed each other and exchanged genes,” Vershinina sums up.
Continuity is evident
In detail it shows: In the middle Pleistocene, shortly after the two lines diverged, the movement was mainly from east to west. In the second crossing period in the late Pleistocene there were movements in both directions, but mainly from west to east, according to the clues in the genome. “The usual view was that the horses differentiated into separate species once they were in Asia, but our results now show that there was continuity between populations,” says co-author Ross MacPhee of the American Museum of Natural History .
In these results, the scientists therefore see an importance for the handling of today’s wild horses in the USA, which are descendants of the domestic horses introduced by the Europeans. Because of this background, some people do not consider them part of the native fauna of North America, but rather an invasive species. With reference to their results, the researchers speak out against this negative view: “There were horses in North America for a long time, and they occupied an ecological niche here,” says Vershinina. “They died out about 11,000 years ago – but that is a short time in evolutionary terms. Therefore, today’s wild North American horses can be seen as reintroduced, rather than invasive, ”the scientist believes.
Source: University of California in Santa Cruz, specialist article: Molecular Ecology, doi: 10.1111 / mec.15977